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VLBI Radio Telescopes in Radio Astronomy

This meeting covered the fundamentals of Very Long Baseline Interferometry (VLBI) radio telescopes and their application in imaging black holes, including demonstrations of wave interference using various types of waves. The discussion then delved into the history and principles of radio astronomy, including antenna calibration and the use of interferometry for precise measurements. Finally, the meeting focused on the Event Horizon Telescope project, its successful imaging of black holes, and future plans for expanding the project's capabilities.

Ted Kochanski presented and first he explains the basics of Very Long Baseline Interferometry (VLBI) radio telescopes and their use in imaging black holes. He discusses the importance of precision timing, data collection, and correlation processing in VLBI. Ted then introduces the concept of interference, starting with Christian Huygens' wave theory of light and Thomas Young's double-slit experiment. He describes how Young demonstrated interference patterns with water waves and light, laying the foundation for understanding wave interference in radio astronomy.

A video shows a college lecturer demonstrating water waves created by two tappers, explaining how nodal lines form where the water remains still. He then transitions to demonstrate the same concept with sound waves using two loudspeakers placed 1.5 meters apart.

The lecturer explains the concept of sound wave interference using two loudspeakers producing a 3,000 Hz tone. He calculates the angles and distances between maxima and minima for different positions in the lecture hall. In the video, his class then participates in a practical demonstration, with students standing up and moving their heads slowly to find locations of silence. The instructor points out that the separation between loud sound and silence is about 19 cm for those 5 meters away, increasing to 38 cm at 10 meters, and 60 cm at the back of the hall. Many students successfully identify areas of near-silence, confirming the theoretical calculations.

The lecturer demonstrates the phenomenon of light interference using a laser beam and a slide with two narrow slits. He explains that when light passes through these slits, it creates an interference pattern on a distant screen, with alternating bright and dark spots. The lecturer calculates the expected angles and distances between the maxima, predicting a separation of about 7.2 cm. He then shows the actual interference pattern, confirming his calculations and emphasizing the remarkable fact that light plus light can create darkness due to wave interference. The lecturer notes that this demonstration illustrates Huygens' principle and the wave nature of light.

The instructor demonstrates interference patterns using various types of waves, including sound, water, laser light, and radar. He shows that the same equations apply to all these wave types, reinforcing the wave nature of light. The demonstration includes a 10 GHz transmitter setup to show radar interference, with calculations for maximum and minimum interference points. The instructor emphasizes the importance of understanding interference by highlighting that blocking one transmitter reduces intensity by a factor of four, not two.

Ted introduces Alan Rogers, a research scientist at Haystack Observatory who taught him about radio and radar astronomy. He then discusses the history and fundamentals of radio astronomy, starting with its inception by Carl Jansky in 1932. Ted explains the challenges of radio interference and the importance of the 1400-1427 MHz band for radio astronomy. He also describes early radio sky mapping efforts by Grote Reber and presents a modern image of the radio sky at 408 MHz, highlighting the prominence of the Milky Way and other strong sources.

The speaker explains the concept of temperature in radio astronomy and its importance in calibrating antennas. They discuss how antenna gain is related to surface area and wavelength, and present formulas for antenna efficiency and beam width. The speaker then describes a simple method for calibrating radio astronomy antennas by comparing the signal strength when pointing at the sky versus an absorber, introducing the concept of the Y factor and system temperature.

The discussion covers various aspects of radio astronomy and interferometry. It explains how Vera Rubin's measurements of galactic rotation curves led to the concept of dark matter. The speaker describes the use of very long baseline interferometry (VLBI) for precise measurements of Earth's parameters and high-resolution observations of celestial objects. They explain how interferometers work, including the use of two antennas to form a single baseline and the importance of closure measurements with three antennas. The speaker also mentions plans to deploy a system in the Catano Valley in Oregon, which is considered as quiet as a site in Western Australia for radio observations.

The discussion focuses on the Event Horizon Telescope (EHT) project and its goal to image black holes. Ted mentions that the EHT team, led by Shep Doleman, began to see that they could actually resolve black holes. The conversation then transitions to a lecture by Doleman, which provides background on Einstein's theory of general relativity and its implications for understanding gravity and black holes. Doleman explains how Einstein's predictions were confirmed through observations of Mercury's orbit and light bending during a solar eclipse, leading to a fundamental shift in our understanding of the universe.

The speaker discusses the characteristics of white dwarfs, neutron stars, and black holes, explaining their increasing density and gravitational effects. They focus on supermassive black holes at the centers of galaxies, describing how they power jets of material and emit intense radiation. The speaker then explains the challenges of imaging a black hole, including the need to observe at specific wavelengths to overcome scattering effects. They mention the successful imaging of the black hole at the center of the Milky Way and M87, highlighting the importance of these observations for testing Einstein's theory of relativity.

Ted presents a series of images from the Event Horizon Telescope (EHT) project, showing various observation sites and recent results. He highlights the polarization image of a supermassive black hole, which reveals magnetic field structures. Ted also discusses future plans for the EHT, including more baselines, longer baselines, and changes in frequencies. The group agrees to continue the discussion in two weeks during a potpourri session, as new material from a recent meeting is expected to be released soon.